I just came across a bounty added to an old question on SO. That question has hundreds of upvotes and stars, nearly 100K views and an accepted answer with a score of over 1K.

Why would someone put a bounty on such a question? It's highly unlikely that anyone, save the person who posted the bounty, will even scroll down enough to see any new answers, never mind upvote them after having read the accepted answer (which has had tons of edits and is a real gem, as you probably won't be surprised to hear).

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Bounties are also often used to "tip" users that provide help one needs. For instance, I had a problem I thought I'd have to ask a new question for, but found not only an existing question, but an answer that so thouroughly resolved my issue that I wanted to give more than just one upvote. So I applied a bounty and awarded it later.

Another possibility is that someone posted the bounty in an attempt to prevent the question from getting closed. Close votes cannot be cast while a bounty is in effect.

Moderators can refund and cancel bounties, though, so it only works for protection against closures by community.

Looking at the question, it looks like Adam Davis' theory is more likely. The question is a community wiki now, so upvotes do not award reputation. It seems that the bounty owner wanted to reward the poster with some reputation and did so with a bounty.